[llvm-dev] vectorize.enable

[Michael Kruse via llvm-dev]

Am Mi., 2. Okt. 2019 um 15:56 Uhr schrieb Finkel, Hal J. <hfinkel at anl.gov>:
> > It's done by the WarnMissedTransformation and just looks for
> > transformation metadata that is still in the IR after all passes that
> > should have transformed them have ran. That is, it does not know why
> > it is still there -- it could be because the LoopVectorize pass is not
> > even in the pipeline -- and we cannot be more specific in the message.
> > However, -Rpass-missed=loop-vectorize may give more information.
>
> As I recall, there is some trade-off here because it's hard for a
> transformation to know that it's last - either the last run of that
> particular transformation in the pipeline or the last transformation in
> the pipeline that can service a particular transformation request (and
> this is especially true if there are multiple, separated pipelines
> involved, such as in LTO). This is why we did not have transformations
> warn if they can't perform the requested transformation for structural
> reasons - maybe they will be able to later. However, we also should
> improve the diagnostics in these cases.

This was not the only consideration. With ordered transformations,
such as vectorize after unroll-and-jam, the LoopVectorize does not
even have a chance to analyze the code since it is located after the
LoopUnrollAndJam pass. We would still warn that vectorization has not
been performed.


> I recommend that we consider taking a kind of delayed-diagnostic
> approach. When a pass cannot perform the requested transformation, it
> records some rationale into the metadata. That rationale can be reported
> by WarnMissedTransformation, if available, to make the diagnostic more
> helpful. If the transformation is later actually performed, then the
> extra information is discarded along with the transformation metadata
> itself.

I like the idea, but I am not sure how helpful are messages such as
"The exiting block is not the loop latch" or "Cannot identify array
bounds" are to the end user. It would still be an improvement.

If there is no diagnostic metadata, do we keep emitting the current
message? If there already is an explanation metadata, does the new one
override the old one or is it appended?

We could also just a hint to the diagnostic such as
"-Rpass=loop-vectorize may provide more information".

Michael